Patent GB-646519-A (GB)
Title: Improvements relating to telephone systems
IPC: H04Q3/42
Filed: 1948-02-27
Abstract: 646,519. Automatic exchange systems. SIEMENS BROS. & CO., Ltd., and LONG, D. P. Feb. 27, 1948, No. 6190. [Class 40 (iv)] In an arrangement for dealing with revertive party-line calls, after the calling party has broken the loop on receiving a busy signal, two check tests of the wanted line are made in timed sequence to determine whether the conditions that follow are appropriate to a revertive call, and if so the final selector is held and the wanted party is rung. As described, the final selector, Fig. 1, which is of the single-motion type controlled by marking from a common circuit, Fig. 2, may be used either solely to give access to multi-party-lines requiring code ringing or solely to give access to ordinary lines and two-party-lines, an appropriate feed-bridge circuit being associated in each case. Setting of final selector and initial test of wanted line. Upon seizure, line relay A in the control circuit energizes over a loop in series with PP, or to earth at a preceding stage if a discriminating signal is applied. Release relay B and its auxiliaries BA, BB operate, followed by dialling relay C and coupling relay K of the selector, whereupon wiper-switching relay WS energizes if PP is operative. The incoming impulse trains set digit switches DA, DB, change-over relay PA being connected up by the relapse of C afters the first train. Wipers of DA, DB in series apply markings in the banks of both wipers WY, XZ of the selector, the same markings resulting from a pair of tens digits, but only one of wipers WY, XZ is made effective by its connection over DA5 to relay T. The selector is started by the second relapse of C and is stopped by the operation of T when the marked position is reached. The wanted line is now tested by relay TT, and if idle, this relay energizes and brings about the operation of switching relay H and the release of the control circuit. Wire BW is earthed to switch in the feed-bridge circuit and wire CCW is connected to wiper CCC for the control of code ringing. Busy condition and testing for revertive call. If TT fails to energize, busy relay BT comes up slowly, holds K, BA, BB and PP, if operated, independently of B and connects up BS to send a busy signal. When the calling party breaks the loop, A, B and PA fall back and initiate the slow release of BT and the release of the preceding switches. Battery instead of earth is now connected to TT to make the first check test of the line, which for a revertive call should continue to test busy long enough for TT and TD to operate in turn and then to test idle in time for further operations to occur before BT falls back. Assuming that TT, TD operate, on the subsequent release of TT, when busy earth disappears, revertive call relay TE energizes in series with TD and reconnects earth to TT, which makes the second check test and should operate to idle potential. Relays TB, TC then energize and connect up switching relay H, battery being connected by TD to wire RCW to signal a revertive call to the feed-bridge circuit. The ultimate relapse of BT releases the control circuit and also the final selector if the above sequence of operations has not taken place. Feed-bridge circuit, Fig. 3. This is associated with a final selector giving access to multiparty-lines and is of the repeating-coil type. On a non-revertive call, relays HA, R energize over wire BW when the final selector switches through, followed by release relay FBB. The metallic byepass is opened and the calling loop is connected to line relay FBA which energizes and holds FBB. When RS is operated by earth on ring start wire RSW, CC is impulsed over wire CCW and code ringing from wire RGW is applied to the + wire, ring-trip relay F being connected to the - wire and corresponding ringing tone being sent to the calling party from wire RTW. Relay F operates upon reply and disconnects R to complete the connection. Release is initiated by the calling party and a thermal relay TH ensures release if the called party fails to clear. On a revertive call, FBA does not energize as the calling party has cleared, but HR operates over wire RCW, holds FBB and puts RR under the control of wire RSW which is earthed periodically, so that each code ring is preceded by a ringing pulse on the - wire as an indication to the calling party should his bell be connected to that wire. Upon reply, both parties are fed over back-bridge relay D, which controls release. According to the Provisional Specification, the circuit may be such that on toll calls earth on the incoming P wire causes the maintenance of the metallic byepass.